[FragmentItem] Add NGInlineBackwardCursor

Hit-testing requires traversing siblings in the reverse
order, which is not easy for either |NGPaintFragment| nor
|NGFragmentItem|. Currently |NGBoxFragmentPainter| copies all
siblings to a list and iterate the list in the reverse order.
This is not very efficient, but hit-testing is the only user
today, and seems to be fast enough.

In order to abstract the operation, this patch adds
|NGInlineBackwardCursor|. This is a separate class, to avoid
increasing stack size for every use of |NGInlineCursor|.

We may revisit this if more users appear.

Bug: 982194
Change-Id: Ie07564dccbcfc3ff7222392fc6f483c95e0579eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939152Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719568}

third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor.cc

@@ -1011,6 +1011,51 @@ void NGInlineCursor::MoveToPreviousSiblingPaintFragment() {
   NOTREACHED();
 }
 
+NGInlineBackwardCursor::NGInlineBackwardCursor(const NGInlineCursor& cursor) {
+  DCHECK(cursor);
+  if (cursor.root_paint_fragment_) {
+    for (NGInlineCursor sibling(cursor); sibling; sibling.MoveToNextSibling())
+      sibling_paint_fragments_.push_back(sibling.CurrentPaintFragment());
+    current_index_ = sibling_paint_fragments_.size();
+    if (current_index_)
+      current_paint_fragment_ = sibling_paint_fragments_[--current_index_];
+    return;
+  }
+  if (cursor.IsItemCursor()) {
+    for (NGInlineCursor sibling(cursor); sibling; sibling.MoveToNextSibling())
+      sibling_item_iterators_.push_back(sibling.item_iter_);
+    current_index_ = sibling_item_iterators_.size();
+    if (current_index_)
+      current_item_ = sibling_item_iterators_[--current_index_]->get();
+    return;
+  }
+  NOTREACHED();
+}
+
+NGInlineCursor NGInlineBackwardCursor::CursorForDescendants() const {
+  if (const NGPaintFragment* current_paint_fragment = CurrentPaintFragment())
+    return NGInlineCursor(*current_paint_fragment);
+  // TODO(kojii): Implement for items.
+  NOTREACHED();
+  return NGInlineCursor();
+}
+
+void NGInlineBackwardCursor::MoveToPreviousSibling() {
+  if (current_index_) {
+    if (current_paint_fragment_) {
+      current_paint_fragment_ = sibling_paint_fragments_[--current_index_];
+      return;
+    }
+    if (current_item_) {
+      current_item_ = sibling_item_iterators_[--current_index_]->get();
+      return;
+    }
+    NOTREACHED();
+  }
+  current_paint_fragment_ = nullptr;
+  current_item_ = nullptr;
+}
+
 std::ostream& operator<<(std::ostream& ostream, const NGInlineCursor& cursor) {
   if (cursor.IsNull())
     return ostream << "NGInlineCursor()";

third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor.h

@@ -348,6 +348,35 @@ class CORE_EXPORT NGInlineCursor {
 
   // Used in |MoveToNextForSameLayoutObject()| to support culled inline.
   const LayoutInline* layout_inline_ = nullptr;
+
+  friend class NGInlineBackwardCursor;
+};
+
+class CORE_EXPORT NGInlineBackwardCursor {
+  STACK_ALLOCATED();
+
+ public:
+  NGInlineBackwardCursor(const NGInlineCursor& cursor);
+
+  NGInlineCursor CursorForDescendants() const;
+
+  explicit operator bool() const {
+    return current_paint_fragment_ || current_item_;
+  }
+
+  const NGFragmentItem* CurrentItem() const { return current_item_; }
+  const NGPaintFragment* CurrentPaintFragment() const {
+    return current_paint_fragment_;
+  }
+
+  void MoveToPreviousSibling();
+
+ private:
+  Vector<const NGPaintFragment*, 16> sibling_paint_fragments_;
+  Vector<NGInlineCursor::ItemsSpan::iterator, 16> sibling_item_iterators_;
+  const NGPaintFragment* current_paint_fragment_ = nullptr;
+  const NGFragmentItem* current_item_ = nullptr;
+  wtf_size_t current_index_;
 };
 
 CORE_EXPORT std::ostream& operator<<(std::ostream&, const NGInlineCursor&);

third_party/blink/renderer/core/layout/ng/inline/ng_inline_cursor_test.cc

@@ -7,6 +7,7 @@
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/blink/renderer/core/layout/layout_text.h"
+#include "third_party/blink/renderer/core/layout/ng/inline/ng_fragment_item.h"
 #include "third_party/blink/renderer/core/layout/ng/inline/ng_inline_node_data.h"
 #include "third_party/blink/renderer/core/layout/ng/ng_layout_test.h"
 
@@ -35,6 +36,40 @@ class NGInlineCursorTest : public NGLayoutTest,
     return list;
   }
 
+  Vector<String> SiblingsToDebugStringList(const NGInlineCursor& start) {
+    Vector<String> list;
+    for (NGInlineCursor cursor(start); cursor; cursor.MoveToNextSibling())
+      list.push_back(ToDebugString(cursor));
+    return list;
+  }
+
+  // Test |MoveToNextSibling| and |NGInlineBackwardCursor| return the same
+  // instances, except that the order is reversed.
+  void TestPrevoiusSibling(const NGInlineCursor& start) {
+    if (start.IsPaintFragmentCursor()) {
+      Vector<const NGPaintFragment*> forwards;
+      for (NGInlineCursor cursor(start); cursor; cursor.MoveToNextSibling())
+        forwards.push_back(cursor.CurrentPaintFragment());
+      Vector<const NGPaintFragment*> backwards;
+      for (NGInlineBackwardCursor cursor(start); cursor;
+           cursor.MoveToPreviousSibling())
+        backwards.push_back(cursor.CurrentPaintFragment());
+      backwards.Reverse();
+      EXPECT_THAT(backwards, forwards);
+      return;
+    }
+    DCHECK(start.IsItemCursor());
+    Vector<const NGFragmentItem*> forwards;
+    for (NGInlineCursor cursor(start); cursor; cursor.MoveToNextSibling())
+      forwards.push_back(cursor.CurrentItem());
+    Vector<const NGFragmentItem*> backwards;
+    for (NGInlineBackwardCursor cursor(start); cursor;
+         cursor.MoveToPreviousSibling())
+      backwards.push_back(cursor.CurrentItem());
+    backwards.Reverse();
+    EXPECT_THAT(backwards, forwards);
+  }
+
   String ToDebugString(const NGInlineCursor& cursor) {
     if (cursor.IsLineBox())
       return "#linebox";
@@ -510,33 +545,27 @@ TEST_P(NGInlineCursorTest, NextForSameLayoutObject) {
   EXPECT_THAT(list, ElementsAre("abc", "", "def", "", "ghi"));
 }
 
-TEST_P(NGInlineCursorTest, NextSibling) {
+TEST_P(NGInlineCursorTest, Sibling) {
   // TDOO(yosin): Remove <style> once NGFragmentItem don't do culled inline.
   InsertStyleElement("a, b { background: gray; }");
   NGInlineCursor cursor =
       SetupCursor("<div id=root>abc<a>DEF<b>GHI</b></a>xyz</div>");
   cursor.MoveToFirstChild();  // go to "abc"
-  Vector<String> list;
-  while (cursor) {
-    list.push_back(ToDebugString(cursor));
-    cursor.MoveToNextSibling();
-  }
+  Vector<String> list = SiblingsToDebugStringList(cursor);
   EXPECT_THAT(list, ElementsAre("abc", "LayoutInline A", "xyz"));
+  TestPrevoiusSibling(cursor);
 }
 
-TEST_P(NGInlineCursorTest, NextSibling2) {
+TEST_P(NGInlineCursorTest, Sibling2) {
   // TDOO(yosin): Remove <style> once NGFragmentItem don't do culled inline.
   InsertStyleElement("a, b { background: gray; }");
   NGInlineCursor cursor =
       SetupCursor("<div id=root><a>abc<b>def</b>xyz</a></div>");
   cursor.MoveToFirstChild();  // go to <a>abc</a>
   cursor.MoveToFirstChild();  // go to "abc"
-  Vector<String> list;
-  while (cursor) {
-    list.push_back(ToDebugString(cursor));
-    cursor.MoveToNextSibling();
-  }
+  Vector<String> list = SiblingsToDebugStringList(cursor);
   EXPECT_THAT(list, ElementsAre("abc", "LayoutInline B", "xyz"));
+  TestPrevoiusSibling(cursor);
 }
 
 TEST_P(NGInlineCursorTest, NextSkippingChildren) {